L7 - Virus uncoating

Question 1

What is the role of the ubiquitin ligase system in the cell?

Answer 1

It conjugates the small peptide ubiquitin (76 amino acids) to target proteins, marking them for degradation by the proteasome.

Question 2

How do microtubules contribute to intracellular movement?

Answer 2

They serve as polarised highways where the dynamic + end is used for polymerisation/depolymerisation, while the – end is anchored at the microtubule organising centre (MTOC).

Question 3

Which motor proteins facilitate movement along microtubules, and in what directions do they travel?

Answer 3

Kinesin moves cargo away from the MTOC, and dynein (with dynactin) moves cargo towards the MTOC.

Question 4

What function do nuclear pore complexes (NPCs) serve in the cell?

Answer 4

NPCs act as gatekeepers for the nucleus, allowing passive diffusion for small proteins and mediating active transport for larger molecules via import and export factors.

Question 5

What type of genome does adenovirus have, and where does its replication occur?

Answer 5

Adenovirus possesses a non-enveloped, linear double-stranded DNA genome of about 36,000 bp, and it replicates in the nucleus.

Question 6

Which major structural proteins form the adenovirus capsid?

Answer 6

The capsid is mainly composed of hexon and penton base proteins, supported by several cement proteins.

Question 7

Why is adenovirus often used in gene therapy and vaccine delivery?

Answer 7

Due to its well-characterised nature, ability to deliver genetic material, and established use as an anti-cancer agent and vaccine vector.

Question 8

What is the significance of adenovirus being assembled as a “precursor” virus?

Answer 8

The precursor virus is formed in the nucleus and requires processing by a virally coded protease (AVP) to uncoat properly upon entry into a new cell.

Question 9

How does adenovirus enter the host cell after attachment?

Answer 9

It is internalised via clathrin-mediated endocytosis into an endosome.

Question 10

What triggers the initial disassembly of adenovirus within the endosome?

Answer 10

The drop in pH during endosome trafficking initiates partial degradation of the virus, leading to the release of mature protein VI.

Question 11

What role does protein VI play in adenovirus uncoating?

Answer 11

Protein VI induces negative curvature in the endosomal membrane, lysing it to allow the virus to escape into the cytoplasm.

Question 12

Why is the ubiquitination of protein VI important?

Answer 12

Ubiquitination of protein VI after endosomal escape is required to facilitate microtubule-mediated transport of the partially degraded virus.

Question 13

How is the partially degraded adenovirus transported within the cell after endosomal escape?

Answer 13

It associates with microtubules via kinesin (attaching to the penton base) and dynein (attaching to the hexon), allowing it to ‘surf’ towards the nuclear pore complex.

Question 14

What occurs when adenovirus reaches the nuclear pore complex (NPC)?

Answer 14

The virus docks with components Nup214 and Nup358, and ubiquitin ligases and the proteasome are recruited to further dismantle the capsid.

Question 15

Which host ubiquitinase is involved at the NPC, and what does it do?

Answer 15

The ubiquitinase MIB1 triggers degradation of viral protein V, which is necessary for releasing the viral genome.

Question 16

How does viral protein V contribute to genome release?

Answer 16

Protein V appears to affect the mechanical properties of the degrading virus particle, reversing the genome condensing effect of protein VII and aiding its import into the nucleus.

Question 17

What structural forms can influenza particles take, and how is the viral genome organised?

Answer 17

Influenza particles can be round or filamentous; the genome segments are clustered at one end, associated with the matrix protein M1.

Question 18

Describe the initial entry process of influenza virus into the host cell.

Answer 18

Influenza enters via micropinocytosis or endocytosis, with trafficking along microtubules or actin filaments; as the endosome matures, its pH drops.

Question 19

What role does the M2 protein play during influenza uncoating?

Answer 19

M2 allows hydrogen ions to enter the viral particle, leading to the dissociation of M1 from the viral ribonucleoproteins.

Question 20

How does the host cell contribute to influenza virus uncoating after endosomal fusion?

Answer 20

Exposure of unanchored ubiquitin chains upon fusion recruits HDAC6, which, along with molecular motors and aggresome components, exerts a shearing force to disassemble the viral interior.

Question 21

What is the role of E3 ubiquitin ligase Itch in influenza uncoating?

Answer 21

It polyubiquitinates matrix protein M1, contributing to the uncoating process.

Question 22

How does TNPO1 (transportin 1) assist influenza virus uncoating?

Answer 22

TNPO1 associates with M1 proteins, promoting their dissociation from the viral genome complexes.

Question 23

What role do importins play after influenza uncoating?

Answer 23

Importins alpha and beta bind to the newly released viral ribonucleoproteins, facilitating their transport into the nucleus via the NPC.

Question 24

How do adenovirus and influenza virus uncoating processes compare?

Answer 24

Both viruses utilise host cell functions, particularly the ubiquitin system, and rely on microtubule-mediated transport and NPC interaction to deliver their genomes to the nucleus.

Question 25

What common host system is exploited by both adenovirus and influenza during uncoating?

Answer 25

Both viruses utilise components of the ubiquitin ligase system to tag viral proteins for degradation, which is critical for genome release.

Question 26

What makes virus uncoating a complex process?

Answer 26

It involves multiple coordinated steps—including capsid disassembly, membrane disruption, intracellular transport, and nuclear import—each relying on distinct host factors and viral proteins.

Question 27

Why is understanding virus uncoating important for virology?

Answer 27

It reveals potential targets for antiviral strategies and improves our overall understanding of virus–host interactions during the early stages of infection.

Question 28

What is the ubiquitin ligase system and what is its role in the cell?

Answer 28

It is a protein-tagging system that conjugates the small peptide ubiquitin (76 amino acids) to lysine residues on target proteins via a cascade of E1 (activating), E2 (conjugating), and E3 (ligating) enzymes. This modification can signal for degradation by the proteasome, alter protein location, or regulate activity.

Question 29

What is the structure and polarity of microtubules and how does this relate to intracellular transport?

Answer 29

Microtubules are polarised tubulin polymers with a dynamic + end that grows/shrinks and a stable – end anchored at the microtubule organising centre (MTOC). Kinesin moves cargo toward the + end (away from MTOC), and dynein (with dynactin) moves cargo toward the – end (toward MTOC).

Question 30

What roles do kinesin and dynein play in intracellular transport?

Answer 30

Kinesin mediates anterograde (outward) transport, attaching to cargo like vesicles or viruses and walking toward the + end. Dynein facilitates retrograde (inward) transport, using dynactin to link cargo and transport toward the – end at the MTOC.

Question 31

How do nuclear pore complexes (NPCs) regulate nucleocytoplasmic transport?

Answer 31

NPCs enable passive diffusion of small molecules and actively transport large proteins via recognition of nuclear localisation signals (NLSs) by importins, powered by a Ran GTPase cycle. They also serve as docking points for some viruses during genome delivery.

Question 32

How does adenovirus initiate infection and enter the host cell?

Answer 32

Adenovirus binds to host cell receptors, is internalised via clathrin-mediated endocytosis, and trafficked in endosomes. Acidification of the endosome induces partial capsid degradation and release of protein VI.

Question 33

What is the function of protein VI in adenovirus infection?

Answer 33

Protein VI disrupts the endosomal membrane by inducing negative curvature, facilitating endosomal escape. Its subsequent ubiquitination enables interaction with microtubules for transport.

Question 34

Why is ubiquitination of protein VI important?

Answer 34

It allows recognition by motor proteins, linking the partially disassembled virion to the microtubule network for cytoplasmic transport toward the nucleus.

Question 35

How is adenovirus transported inside the cytoplasm post-endosomal escape?

Answer 35

The virus binds microtubules via kinesin (through penton base) for anterograde and dynein (through hexon) for retrograde transport. It 'surfs' along microtubules toward the nuclear pore complex.

Question 36

What happens when adenovirus reaches the nuclear pore complex (NPC)?

Answer 36

The virus docks via Nup214 and Nup358. Host ubiquitin ligases (notably MIB1) and the proteasome degrade viral capsid proteins such as protein V, allowing genome release.

Question 37

What is the role of MIB1 in adenovirus uncoating at the NPC?

Answer 37

MIB1 polyubiquitinates viral protein V, targeting it for proteasomal degradation. This step is necessary to release the viral genome and allow its import.

Question 38

How does protein V regulate adenovirus genome release?

Answer 38

It stabilises the virion and restricts genome expansion. Upon degradation by MIB1/proteasome, the condensing effect of protein VII is reversed, allowing genome entry into the nucleus.

Question 39

How does influenza virus enter host cells and traffic internally?

Answer 39

It enters by endocytosis or micropinocytosis. Endosomes mature and acidify during intracellular transport via microtubules and actin, preparing the virus for uncoating.

Question 40

What is the role of M2 ion channel in influenza virus uncoating?

Answer 40

M2 facilitates proton influx into the virion as the endosome acidifies, causing matrix protein M1 to dissociate from viral ribonucleoproteins (vRNPs), allowing uncoating.

Question 41

How does the drop in endosomal pH contribute to influenza uncoating?

Answer 41

It enables hemagglutinin conformational change for membrane fusion and activates M2, which destabilises M1–vRNP interactions.

Question 42

How does the host cell machinery facilitate mechanical disassembly of influenza?

Answer 42

Fusion exposes unanchored ubiquitin chains that recruit HDAC6. This enzyme links to dynein motors and aggresome components to apply a shearing force that breaks apart viral interiors.

Question 43

What are aggresomes and what is their role in influenza uncoating?

Answer 43

Aggresomes sequester misfolded proteins and are repurposed by the virus to facilitate mechanical disassembly during uncoating.

Question 44

How does HDAC6 contribute to influenza virus uncoating?

Answer 44

HDAC6 binds unanchored ubiquitin chains exposed post-fusion and links them to motor proteins that physically disrupt the virus.

Question 45

What role does E3 ligase Itch play in influenza virus uncoating?

Answer 45

It polyubiquitinates M1 protein, aiding in destabilising the virion structure and enabling genome release.

Question 46

What is the function of TNPO1 in influenza virus infection?

Answer 46

TNPO1 binds M1 protein, promoting its dissociation from vRNPs and freeing them for nuclear import.

Question 47

How are influenza viral genomes imported into the nucleus after uncoating?

Answer 47

Importins alpha and beta bind vRNPs, guiding them through the NPC using the cell’s nuclear import machinery.

Question 48

How do adenovirus and influenza both exploit the host ubiquitin system?

Answer 48

Both viruses utilise ubiquitin ligases to degrade viral components that restrain the genome, enabling uncoating and nuclear import.

Question 49

What common intracellular transport mechanism do adenovirus and influenza share?

Answer 49

Both viruses use the microtubule network and motor proteins to move toward the nucleus.

Question 50

What makes viral uncoating a complex and coordinated process?

Answer 50

It involves multiple timed steps including capsid breakdown, membrane disruption, motor-driven transport, proteasomal degradation, and nuclear import—all mediated by host factors and viral effectors.

Question 51

Why is studying viral uncoating important?

Answer 51

It reveals critical host-virus interactions and offers targets for antiviral drugs that could block genome release and infection.